Dissociation of Eu+3 Charge-Transfer State in Y2O2S and La2O2S into Eu+2 and a Free Hole

Abstract

In ${\mathrm{Y}}_2$ ${\mathrm{O}}_2$S: Eu and ${\mathrm{La}}_2$ ${\mathrm{O}}_2$S: Eu, excitation into the charge-transfer states (CTS) of ${\mathrm{Eu}}^{+3\mathrm{}}$ leads partially to the dissociation of the CTS into ${\mathrm{Eu}}^{+2\mathrm{}}$ and a free hole which may subsequently be trapped. The extent of CTS dissociation is measured both by the storage induced by a known CTS excitation dosage and by the slow-rise transients of the ${}_{}{}^5{}_{}{}^{}D$ emissions at the onset of CTS excitation. Both measurements give an activation energy for CTS dissociation of 900 ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{Y}}_2$ ${\mathrm{O}}_2$S and 1300 ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{La}}_2$ ${\mathrm{O}}_2$S, and a rate constant of ∼ ${10}^{13.5}$ ${\sec }^{-1\mathrm{}}$ for both hosts. The steady-state emission intensity undergoes a gradual superlinear transition with excitation intensity. The time constant of the slow-rise transient decreases with excitation intensity. The phosphorescence intensity is smaller than that of the slow-rise transient and increases sublinearly with excitation intensity. These kinds of behavior are all explained with the model of the CTS dissociating into ${\mathrm{Eu}}^{+2\mathrm{}}$ and a free hole, with subsequent hole trapping and ${\mathrm{Eu}}^{+2\mathrm{}}$-trapped-hole nonradiative recombination. In this model, the ${\mathrm{Eu}}^{+2\mathrm{}}$ concentration at steady state increases as the $\frac{1}{2}$ to $\frac{1}{3}$ power of the excitation intensity.